Speaking to a group of Northwestern University marketing students, Yum! Brands Chief Public Affairs Officer Jonathan Blum shared the story of recent Taco Bell promotion flop that shows the difficulty the chain has had turning social media into a viable business model. Said Blum: “We haven’t even been able to give away the food, never mind figure out how to sell it online.”

Over the course of a year, the number of friends on Taco Bell’s Facebook page rocketed from 500,000 to 6 million fans. Sounds great. Then, in the middle of an ongoing unflattering lawsuit about the quality of its beef, Taco Bell decided to offer those 6 million fans a free taco — no strings attached. They didn’t need to buy anything. They were already Facebook fans, which means they had already paid the very minor costs of “liking” Taco Bell. It was an offer from a company that Blum says wanted to tell its fans, hey, come and get a free taco.

Two hundred thousand people did. Almost 97 percent on passed on free grub they supposedly “liked.”

Cases like that explain why the bulk of Taco Bell’s marketing budget goes to television (and some radio) ads. Social media remains a small part of the budget because the company hasn’t figured out how, in Blum’s words, to use it to “make the cash register ring.”